There is known an analysis method of thoroughly visualizing the expression amount of a protein expressed in a tumor tissue or the like based on a mass signal intensity by using a Matrix Assisted Laser Desorption/Ionization (MALDI) or a Time of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS).
In the measurement of a biological sample by the MALDI or TOF-SIMS, a sample constituent is detected in an ionized state. In particular, the sample constituent is detected as a protonated molecule in most cases.
In the measurement of a biological sample by the MALDI or TOF-SIMS, in order to enhance the ionization efficiency of a constituent, a method of supplying an aqueous solution of a matrix, an alkali metal salt, or an acidic substance to a sample by spraying or dropping has been performed. For example, the inventors of the present invention found that the ionization efficiency of a sample is enhanced in the TOF-SIMS measurement by dropping an acidic substance aqueous solution to the sample (PTL 1).
On the other hand, there is also known a method of promoting protonation to a sample constituent by using water contained in the sample or by providing water to the sample from outside, instead of using a matrix or an acidic substance.
NPL 1 discloses a method of measuring a fracture cross-section of sample suspensions which are prepared by sonicating and freezing in H2O.
NPL 2 discloses a method of allowing a sample to adsorb water by freezing the sample in a water atmosphere to generate a protonated molecule.